The term mystery pitcher was coined by Philadelphia Inquirer baseball columnist Jayson Stark. It refers to a position player who comes in to pitch in the late innings of a blowout loss. Sometimes, a mystery pitcher is forced to enter an extra inning game when the bullpen has been emptied of available pitchers. The most recent mystery pitchers to win a game were Brent Mayne in 2000 and Wilson Valdez in 2011. Stevie Wilkerson also made history as a mystery pitcher when he became the first to record a save in 2019. They are also sometimes known as "joke pitchers".
The use of mystery pitchers was very rare until the 1980s, when certain managers began to use the strategy more often as a way to spare their bullpen. This was just an uptick, however, but in the 2010s, the strategy really took off, as there was no longer any stigma attached to it (in earlier times, managers were often accused of making a mockery of the game by their peers or by sportswriters when they resorted to the strategy). For example, from 1969 to 1978, the most instances of the use of the strategy in any season was 3; from 1986 to 1991, there were at least 8 such instances every year, but there was only one season between 1992 and 2008 in which there were as many as 10 (that was 2004, with exactly 10). However, from 2012 onward, the usage blew up, going from 11 in 2008 to 24 in 2015 to an unprecedented 57 in 2018, a total which was shattered in 2019.
Major League Baseball took notice of this trend and began to examine ways to curb it. In 2019, one of the rules changes proposed was that such pitchers could only be used in a game with a margin of 6 or more runs, or in extra innings. The rule was slated to come into effect in 2020 but was suspended for two seasons due to the upheavals created by the Coronavirus pandemic. It was finally invoked for the first time on June 4, 2022, when manager Dave Roberts of the Los Angeles Dodgers wanted to bring in position player Zach McKinstry to pitch the 9th inning with a 9-4 deficit against the New York Mets. Crew chief C.B. Bucknor intervened to prevent the move, and after Roberts objected, he confirmed with MLB Headquarters in New York that he was indeed applying it correctly, forcing Roberts to bring in Evan Phillips. Phillips was not warmed up, but was given unlimited time to do so, as if the previous pitcher had sustained an injury. In spite of this successful invocation, the rule was unlikely to change the burgeoning use of position players on the mound, given that the situations in which they were being used already almost always conformed to those in the rule.
- Gabe Lacques: "Garbage time, all the time: MLB sees historic spike in position players pitching", USA Today, August 21, 2019. 
- Sarah Langs: "Best performances by position players pitching", mlb.com, May 12, 2020. 
- Mike Petriello: "Behold the glory of position players pitching", mlb.com, November 15, 2020. 
- Joe Reedy (Associated Press): "Umpires bar Dodgers' Roberts from pitching position player", Yahoo! News, June 5, 2022. 
- Brian Wright: "Position player pitching? Not for LA in 9th: Rule prevents Dodgers, down 5 runs, from sending McKinstry to mound vs. Mets", mlb.com, June 5, 2022.